1. Field of the Invention
The present invention relates to a cylindrical magnetic circuit comprising two or more permanent magnets placed annularly and generating a magnetic field inside, and a method of producing such a cylindrical magnetic circuit.
2. Description of the Related Art
In an embodiment for producing a magnetic field, a circuit is known in which two or more permanent magnets are combined to form a cylindrical shape and a magnetic field formed inside is used. The circuit in which two or more permanent magnets are combined is called a magnetic circuit, and examples of using a magnetic field inside a cylindrical magnetic circuit include a dipole-ring magnetic circuit, a six-pole magnetic circuit, and the like.
It is generally preferable for these magnetic circuits to have a smaller external dimension provided so that a desired magnetic field is generated inside. In view of this, columnar magnets having a section in a trapezoid shape are combined annularly, so that a density of the magnets in a space is increased. Further, in order to maintain the annular state, the magnets are fixed to a cylindrical yoke, and it is desirable that the yoke be thin so as to reduce the external dimension.
Conceivable methods for producing a cylindrical magnetic circuit include: a method in which two or more magnets are inserted from the cylindrical end along a direction of a central axis of a round section of the cylindrical yoke (hereinafter referred to as “an axial direction”), and the magnets are annularly placed along the inside of the cylindrical yoke; and a method in which two or more magnets are inserted radially from a side of the cylinder along a cylindrical magnetic circuit to be formed and the magnets are placed annularly. In the method in which the magnets are inserted from the cylindrical end along the axial direction, a member serving as a guide is preferably attached to the yoke, so that the magnets are inserted into the yoke along the guide and then the magnets are fixed to the yoke. On the other hand, in the method in which the magnets are inserted radially from a side of the cylinder, a pair of round or annular members for annularly placing the magnets therebetween is preferably provided first. The magnets are inserted into a space between the round or annular members from a radial outside thereof so as to be placed annularly, and the magnets are temporarily held by the round or annular members in the annular state. A yoke is placed so as to cover the magnets placed annularly, the magnets are fixed to the yoke, and then the round or annular members are removed.
In either of the producing methods, the magnets, in a magnetized state, are inserted. Accordingly, during assembling of the cylindrical magnetic circuit, an attractive or repulsive force works between a magnet to be incorporated and a magnet which has been already incorporated. This force continues working between magnets adjacent to each other after the assembling, and the force also continues working on the yoke holding the magnets. If the strength of the yoke decreases as a result of making the yoke thin, the force may deform the yoke and further change the positional relationship between the magnets depending on a relation between the strength of the yoke and the force working between the magnets, which may make it difficult to obtain a desired magnetic field distribution.